An uninterruptible power supply is used to allow a load to receive power without interruption. To increase capacitance and improve reliability, there has been proposed a power supply system including a plurality of uninterruptible power supplies connected to a load in parallel.
For example, Japanese Patent Laying-Open No. 2009-100618 (PTD 1) discloses a method for controlling a plurality of uninterruptible power supplies in parallel. More specially, each uninterruptible power supply outputs a current and a voltage, which are controlled in accordance with a current that the uninterruptible power supply should supply to the load.
For example, Japanese Patent Laying-Open No. 2005-033890 (PTD 2) discloses a method for controlling a system including a plurality of uninterruptible power supplies. Each uninterruptible power supply includes a synchronous control circuit. The synchronous control circuit receives a voltage from a commercial power supply and converts the received voltage into a current. When the plurality of uninterruptible power supplies have power supply voltages with a phase difference, a reference voltage based on the current is generated for correcting an output voltage. Each uninterruptible power supply corrects its output voltage with reference to the reference voltage.